Microstructured fibres, also known as photonic crystal fibres, photonic bandgap fibres, holey fibres, and hole-assisted fibres are today widely explored due to their potential applications within areas such as optical communications, sensor technology, spectroscopy, and medicine (see e.g. Broeng et al., Optical Fiber Technology, Vol. 5, pp. 305–330, 1999; Broeng et al., Optics Communications, Vol. 156, pp. 240–244, 1998, Broeng et al, Optics Letters, Vol. 25 (2), pp. 96–98, 2000; WO9964903; WO9964904; WO0060390; Birks et al., Electronics Letters, Vol. 31 (22), p. 1941, October 1995; Knight et al., Joumal of the Optical Society of America, A., Vol. 15 (3), p. 748, March 1998; Knight et al., Optical Materials Vol. 11, pp. 143–151, 1999; U.S. Pat. No. 5,802,236; Monro et al., Journal of Lightwave Technology, Vol. 17 (6) p. 1093–1102, 1999; Ferrando et al., Optics Letters, Vol. 24 (5), pp. 276–278, 1999; WO0006506). The fibres are characterized by having a core surrounded by thin, parallel, voids/holes in a background material. The background material is most often a single material such as e.g. silica glass, and the voids/holes commonly contain air or vacuum, but they may also be filled with other glass materials, polymers, liquids, or gasses. Depending on the application, the voids/holes may be periodically or randomly distributed, or they may be distributed in specially designed arrangements incorporating both periodic and non-periodic regions (see e.g. WO9964903; WO9964904; WO0060390; U.S. Pat. No. 5,802,236, Monro et al., Optics Letters, Vol. 25 (4), pp. 206–208, 2000).
Typically, microstructured fibres are fabricated by a method that includes stacking a number of circular capillary tubes and one or more solid rods to form a preform that is drawn in a single or more steps using a conventional fibre drawing tower to yield a final fibre with a micron-scale structure in the cross-section—see e.g. U.S. Pat. No. 5,155,792 or Knight et al. Optical Fiber Communication Conference, paper PD3, 1996. Today, this method represents the most widely employed method for fabricating micro-structured fibres, but other methods have been proposed in the past:                The first microstructured fibres were investigated in the 1970's (see e.g. Kaiser et al., The Bell System Technical Journal, Vol. 53, No. 6, July–August 1974, pp. 1021–1039), and they were of a rib-waveguide type. Typically, the fibre preform was formed by a core rod positioned on a thin, polished plate that was suspended in a thick-walled cladding tube. The preform was normally made from a single material—typically silica—and fibres of this type are also referred to as single material fibres. The method has been improved by providing active control of the pressure in the interior of the fibre preform during drawing to control more accurately the inner structure of the fibre (see e.g. U.S. Pat. No. 4,046,537).        Another method involves drilling of holes into solid glass rods of diameter around 30 mm using an ultra-sonic assisted mechanical drill. The resulting holey canes are afterwards milled on the outside in order to realise six flats, and then drawn into hexagonal capillary tubes. Finally the hexagonal capillary tubes are stacked in a close-packed manner to produce a fibre preform that may be drawn into fibre. The method was proposed by Birks et al. (see Birks et al., Photonic Band Gap Materials, Kluwer 1996) as the first method for fabrication of microstructured fibres that could guide light by photonic bandgap effects.        Yet another method involves fabrication of multi-channelled glass structures using one or more steps of extrusion (see e.g. U.S. Pat. No. 5,774,779) and drawing these glass structures into microstructured fibres using conventional drawing techniques (see e.g. WO0006506).        
Further improved methods for fabricating microstructured fibres have been proposed: methods using preforms incorporating so-called jigs and short-length capillary tubes and/or rods in the cladding or core region—see WO9964903 or WO0060388, respectively.
Furthermore, it is well-known to those skilled in the art of fabricating microstructured optical fibres that improved control of the fibre structure may be obtained by employment of passive and/or active control of pressure in various regions of the fibre preform during drawing to cane and/or final fibre (see e.g. U.S. Pat. No. 4,046,537, EP0810453A1, WO0049436). Finally, it should be mentioned that employment of pressure for fabrication of optical fibres, capillary tubes, and overcladding tubes for fibre preforms is a well-known technique (see e.g. GB2123810A or JP5992940 for fibre fabrication using pressurization over regions inside fibre preforms).
In PATENT ABSTRACTS OF JAPAN, appl. No. 57160845, Yokata et al. describe an optical fibre, produced using a preform comprising a quartz tube having axis-symmetrical opposed side grooves on the outside peripheral surface. Yokata et al. utilizes two grooves for realizing polarization maintaining operation of the fibre. The grooves disclosed by Yokata et al. are positioned in close to a central doped rod. The central doped rod act to form the core of the resulting fibre after drawing of the preform to fibre—in a manner similar to the core of standard optical fibres. The two grooves in the tube acts to form a non-circular shape of the doped core in order for the fibre to exhibit a high birefringence and thereby by polarization maintaining.
In order to obtain fibre that exhibits low birefringence, for example low polarization mode dispersion, PMD, it is a disadvantage to use two axis-symmetrical opposed side grooves close to the fibre core as this results in a distortion of the core region away from a circular symmetry.
In U.S. Pat. No. 4,049,413, French disclosed a method for realizing fibres having diameter fluctuations in the transmitting core. The diameter variations are obtained by etching grooves in the preform for the fibre during fabrication. The grooves are etched using lithographic. processes and are circumscribing the longitudinal axis of the preform.
It is a disadvantage of the method described by French, that it does not provide microstructured optical fibres.
It is a further disadvantage of the method described by French that it does not provide optical fibres that are uniform in the longitudinal direction.
In WO 9957070 Berkey et al. discloses a method of making a glass fibre with axially arying properties. Berkey et al. discloses fibre preforms having four grooves that may be realized using dicing saw or non-contaminating CO2 laser. The grooves are placed in ontact with a doped region acting as the core in the final fibre. The grooves may form air hannels over some limited lengths of the fibre and be collapsed over others. The doped core will serve as a waveguide in both cases, but the waveguiding properties will be different in the section with channels as compared to the sections without.
It is a disadvantage of the method disclosed by Berkey et al. that it does not provide optical fibres that are uniform in the longitudinal direction.
It is a further disadvantage of the method disclosed by Berkey et al. that the grooves are placed in close contact with the doped core. It is a further disadvantage of the method disclosed by Berkey et al. that it does not provide preforms with more than four grooves. It is a further disadvantage of the method disclosed by Berkey et al. that only a single layer of grooves surrounding the core region is provided.
In U.S. Pat. No. 5,907,652, DiGiovanni et al. discloses a method for fabricating air-clad optical fibres. The air-clad layer is realized using a number of capillary tubes that are positioned outside a preform rod that serves as inner cladding and core region in the final fibre. The capillary tubes may be attached to the preform rod by melting their ends to the preform.
It is a disadvantage for the realization of air-clad optical fibre to use capillary tubes, as it may be very time consuming and require a substantial amount of manual work to attach the individual capillary tubes to the preform.
It is an objective of the present invention to provide new preforms or parts thereof for microstructured fibres—in particular for air-clad fibres—as well as new methods for realizing such fibres, where the use of individual capillary tubes is eliminated. It is a further object of the present invention to provide methods of realizing preforms or parts thereof for microstructured fibres that may reduce manual work force or that may be fully automated. It is a further objective of the present invention to provide methods for realizing preforms or parts thereof for microstructured fibres comprising a very large number of air voids (more than 30 voids) in one or more annular layers.
It is a disadvantage of the method of producing fibres of the rib-waveguide type that it is difficult to accurately control the waveguiding properties of the final fibres with respect to single-mode operation, numerical aperture, propagation loss, dispersion, polarisation, etc.
It is a further disadvantage of the method of producing fibres of the rib-waveguide type that it is not suitable for fabricating periodic features in a cross section of the microstructured fibres. The method is therefore not suitable for fabricating microstructured fibres that operate by photonic bandgap effects.
It is an advantage of the method employing stacking of capillary tubes and/or rods that the above-mentioned waveguiding properties in the resulting fibres may be more accurately controlled than for rib-waveguide type fibres.
It is a further advantage of the method employing stacking of capillary tubes and/or rods that it is suitable for fabricating microstructured fibres with periodic features in the fibre cross section. Hence, the method is suitable for fabricating microstructured fibres that operate by photonic bandgap effects.
It is, however, a disadvantage of the methods that utilise stacking of capillary tubes and/or rods that the tubes/rods may slide in position with respect to each other during either preform fabrication or fibre drawing (or both). This degrades reproducibility of the fibre fabrication. This further degrades the possibility of realizing microstructured fibres that operate by photonic bandgap effects.
It is a further disadvantage of the methods employing capillary tubes that the reproducibility is further limited due to the difficulties in fabricating large numbers of capillary tubes with high uniformity in longitudinal direction of both inner and outer diameters. This degrades the possibility of realizing long lengths of microstructured fibres that operate by photonic bandgap effects.
It is a further disadvantage of the methods employing capillary tubes that cleaning of the tubes is troublesome due to difficulties in accessing the inside of capillary tubes with inner diameters of a few millimetres or less. This may degrade fibre performance with respect to propagation losses as a result of possible contamination of the inner surface of the capillary tube that cannot be removed.
It is a disadvantage of the method that utilized ultrasonic-assisted drilling that a large contamination of the preform material usually takes place during fabrication.
It is a further disadvantage of the method that utilized ultrasonic-assisted drilling and milling of flats on the sides of the glass canes that the manufacturing time is long—limiting the use of the method for large-scale production of microstructured fibres.
It is a disadvantage of the methods using close-packed stacking of circular capillary tubes and/or rods that these favour realisation of fibres with hexagonal symmetry in the cross section—and these methods are unsuited for fabrication of fibres with arbitrary cross-sectional designs.
It is an advantage of extrusion-based methods that a high reproducibility can be obtained for the final fibres.
It is a further advantage of extrusion-based methods that a high freedom in the design of final fibre cross section can be achieved simply by manufacturing of die with appropriate design.
It is a further advantage of extrusion-based methods that these are suitable for realizing long lengths of microstructured fibres that operate by photonic bandgap effects.
It is, however, a disadvantage of extrusion-based methods that the die may contaminate the fibre preform during extrusion.
It is a further disadvantage of extrusion-based methods that the glass preforms may experience devitrification during extrusion.
It is an object of the present invention to provide fibre preforms that may be fabricated without the use of extrusion or stacking of capillary tubes and/or rods.
It is a further object of the present invention to provide methods of fabricating microstructured fibres and preforms for microstructured fibres with a high reproducibility and a high degree of design freedom—as for extrusion-based methods—but where risks of contamination are greatly reduced. Especially, it is an object of the present invention to provide methods for fabricating microstructured fibres that utilize laser ablation or laser etching during preform manufacturing.
It is a further object of the present invention to provide fibre preforms for fabrication of microstructured fibres that operate by photonic bandgap effects with the use of only a single capillary tube.
It is a further object of the present invention to provide a method of fabricating microstructured fibres that operate by photonic bandgap effects where only a single capillary tube is used for manufacturing of the fibre preform.
It is a further object of the present invention to provide a method for fabricating microstructured fibres that may be connected to other fibres with low losses.
It is a further object of the present invention to provide preforms for fabrication of microstructured fibres that may be connected to other fibres with low losses.
It is a further object of the present invention to provide microstructured fibres that may be connected to other fibres with low losses.